Question

Find out the order of stability between the species given below.
$C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3$ and $\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3$

• A. $\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3\approx C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3$
• B. $C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3>\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3$
• C. $\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3>C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3$
• D. Cannot predict

Answer 1

The correct option is C $\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3>C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3$

$-CN$ is an electron withdrawing group. Therefore, it will decrease the stability of the carbocation.
As the distance between the electron withdrawing group $(-CN)$ and carbocation carbon increases, the stability of carbocation increases as -I nature of $(-CN)$ is distance dependence.
Therefore, the order of stability of carbocation is:
$\underset{CN}{\underset{|}{C}}{H}_2-C{H}_2-\stackrel{+}{C}H-C{H}_3>C{H}_3-\underset{CN}{\underset{|}{C}}H-\stackrel{+}{C}H-C{H}_3$